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Flea Allergy Dermatitis in Dogs and Cats


Michael W. Dryden

, DVM, PhD, DACVM, Kansas State University

Last full review/revision Apr 2021 | Content last modified May 2021
Topic Resources

Flea allergy dermatitis is an immunologic disease caused by the injection of antigens from the salivary glands of fleas as they feed on the host animal. The primary clinical signs are pruritis and papulocrustous lesions distributed on the lower back, tailhead, and caudal and inner thighs in dogs or pruritis and papular dermatitis found on the face, neck, and back in cats. Diagnosis can be based on clinical signs, finding fleas, and results of intradermal skin testing. Control is based on treatment and prevention of flea infestations, along with supportive care for dermatitis.

Flea allergy dermatitis (FAD) or flea bite hypersensitivity is the most common dermatologic disease of domestic dogs in the US. Cats also develop FAD, which is one of the major causes of feline miliary dermatitis. FAD is most prevalent in the summer, although in warm climates flea infestations may persist throughout the year. In north temperate regions, the close association of pets and their fleas with human dwellings creates conditions that permit a year-round problem. Temperature extremes and low humidity tend to inhibit flea development.

When feeding, fleas inject saliva that contains a variety of histamine-like compounds, enzymes, polypeptides, and amino acids that span a wide range of sizes (40–60 kD) and induce Type I, Type IV, and basophil hypersensitivity reactions. Flea-naive dogs exposed intermittently to flea bites develop either immediate (15 minutes) or delayed (24–48 hours) reactions, or both, and detectable levels of both circulating IgE and IgG antiflea antibodies. Dogs exposed continuously to flea bites have low levels of these circulating antibodies and either do not develop skin reactions or develop them later and to a considerably reduced extent. This could indicate that immunologic tolerance may develop naturally in dogs continually exposed to flea bites. Although the pathophysiology of FAD in cats is poorly understood, similar mechanisms may exist.

Clinical Findings of Flea Allergy Dermatitis in Dogs and Cats

Clinical signs associated with FAD are variable and depend on frequency of flea exposure, duration of disease, presence of secondary or other concurrent skin disease(s), extent of hypersensitivity, and effects of previous or current treatment. Nonallergic animals may have few clinical signs other than occasional scratching due to annoyance caused by flea bites. Those patients that are allergic will typically have a dermatitis characterized by pruritus.

In dogs, the pruritus associated with FAD can be intense and may manifest over the entire body. Classic clinical signs are papulocrustous lesions distributed on the lower back, tailhead, and caudal and inner thighs. Dogs may be particularly sensitive in the flanks, caudal and medial thighs, ventral abdomen, lower back, neck, and ears. Affected dogs are likely to be restless and uncomfortable, spending much time scratching, licking, rubbing, chewing, and even nibbling at the skin. Hair may be stained brown from the licking and is often broken off. Common secondary lesions include areas of alopecia, erythema, hyperpigmented skin, scaling, papules, and broken papules covered with reddish brown crusts. The rump and tailhead areas are typically the first, most evident, areas affected. As FAD progresses and becomes chronic, the areas become alopecic, lichenified, and hyperpigmented, and secondary bacterial and yeast infections occur.

In extremely hypersensitive dogs, extensive areas of alopecia, erythema, and self-trauma are evident. Traumatic moist dermatitis (hot spots) can also develop. As the disease becomes chronic, generalized alopecia, severe seborrhea, hyperkeratosis, and hyperpigmentation may develop.

In cats, clinical signs vary from minimal to severe, depending on the degree of sensitivity. The primary dermatitis is a papule, which often becomes crusted. This miliary dermatitis is typically found on the back, neck, and face. The miliary lesions are not actual flea bites but a manifestation of a systemic allergic reaction that leads to generalized pruritus and an eczematous rash. Pruritus may be severe, evidenced by repeated licking, scratching, and chewing. Cats with FAD can have alopecia, facial dermatitis, exfoliative dermatitis, and “racing stripe” or dorsal dermatitis.

Diagnosis of Flea Allergy Dermatitis in Dogs and Cats

  • Clinical evaluation for hair loss and presence of fleas

  • Intradermal skin testing

A number of factors must be considered in the diagnosis of FAD, including history, clinical signs, presence of fleas or flea excrement, results of intradermal testing, and exclusion of other causes of dermatologic disease.

Clinical signs associated with FAD are variable and depend upon frequency of flea exposure, duration of disease, presence of secondary or other concurrent skin disease, degree of hypersensitivity, and effects of previous or current treatment. Nonallergic animals may have few clinical signs other than occasional scratching due to annoyance of flea bites. Those that are allergic will typically have dermatitis, characterized by pruritus. In dogs, the pruritus associated with FAD can be intense and may manifest over the entire body. Affected dogs may be particularly sensitive in the flanks, caudal and medial thighs, ventral abdomen, lower back, neck, and ears. Chewing, licking, and scratching of these areas will usually be evident. Hair may develop brown staining from the licking and is often broken off. In extremely hypersensitive dogs, extensive areas of alopecia, erythema, and self-trauma are evident.

Most cases are seen in the late summer, corresponding to the peak of flea populations. In these cases, history can be highly suggestive. Age of onset is also important, because FAD does not ordinarily occur before 1 year of age. Visual observation of fleas on the infested pet is helpful in diagnosis. Demonstration to the owner of the presence of fleas or flea excrement is helpful. Slowly parting the hair against the normal lay often reveals flea excrement or the rapidly moving fleas. Flea excrement is reddish black, cylindrical, and pellet- or comma-shaped. Placed in water or on a damp paper towel and crushed, the excrement dissolves, producing a reddish brown color.

Extremely hypersensitive animals are likely to be virtually free of fleas because of excessive self-grooming. In these patients, it is usually difficult to find evidence of fleas, thus making it harder to convince owners of the problem. Use of a fine-toothed flea comb (32 teeth/in.) facilitates finding of fleas and their excrement. Examination of the pet’s bedding for eggs, larvae, and excrement is also useful.

Intradermal skin testing may be used to support a presumptive diagnosis of FAD. Positive immediate reactions are characterized by a wheal 3–5 mm larger in diameter than the negative control. Alternatively, a positive wheal measurement can be defined as a response that is at least equal in diameter to the halfway point between the size of positive and negative control reactions. Observations for an immediate reaction (15–20 minutes) and, if negative, a 24-hour delayed reaction are recommended. The delayed reaction may not be seen as a discrete wheal but rather as a diffuse erythematous reaction. A positive reaction does not conclusively indicate that the clinical condition is FAD—it indicates only that the animal is allergic to the flea antigen, either from present or past exposure. The reliability of intradermal skin testing in cats to diagnose FAD is variable.

Serologic testing of IgE directed against flea-specific salivary antigens can be used to aid in the diagnosis of FAD.

FAD must be differentiated from other causes of dermatologic disease. The presence of fleas or a positive reaction to an intradermal test does not exclude the presence of another dermatologic disease responsible for the clinical signs. In dogs, differential diagnoses include atopic dermatitis, food allergy dermatitis, sarcoptic or demodectic mange, other ectoparasites, and bacterial folliculitis. In cats, other conditions that can result in miliary dermatitis include external parasites (cheyletiellosis, trombiculosis, notoedric mange, and pediculosis), dermatophytosis, drug hypersensitivity, food allergy, atopic dermatitis, bacterial folliculitis, and idiopathic miliary dermatitis.

Treatment and Control of Flea Allergy Dermatitis in Dogs and Cats

  • Control is based on killing the fleas resident on the patient, elimination of the biomass at the home premises, and prevention of future infestations

  • Supportive care may also be necessary in cases of FAD, including when there is subsequent anemia

Flea control measures have changed dramatically over the years. The development of insecticides and insect growth regulators (IGRs) with convenient dosage formulations and prolonged residual activity has dramatically improved owner compliance and helped eliminate recurrent infestations. The goals of flea control are elimination of fleas on pet(s), elimination of existing environmental infestation, and prevention of subsequent reinfestation. The first step is still the elimination of existing pet flea infestations. Elimination of those fleas currently established on the dog or cat is necessary to eliminate pet discomfort.

One common consideration is termed "rate" or "speed" of flea kill on a patient. However, it is important to differentiate between speed of elimination of established infestations and speed of elimination of newly acquired fleas after a product has been applied. When treating a dog or cat with a topically applied formulation, it could take several hours (12–36 hours) until the compound has spread sufficiently or reached sufficient systemic concentrations to eliminate all existing fleas. If a more rapid rate of kill is needed, a flea spray or systemic oral or topical product such as afoxolaner, fluralaner, lotilaner, sarolaner, or spinosad may be desirable.

Several available insecticides provide excellent elimination of established flea infestations on both dogs and cats, including afoxolaner (dogs only), dinotefuran, fipronil, fluralaner (dogs only), imidacloprid, indoxacarb, lotilaner, nitenpyram, sarolaner, selamectin, and spinosad.

The second step is to eliminate the existing infestation in the pet’s environment. This can be accomplished in several ways: 1) topical application of residual insecticides that kill newly acquired fleas (within 24 hours) before they can initiate reproduction; 2) administration of topical, injectable, or oral IGRs to stop flea reproduction; 3) repeated application of insecticides, IGRs, or both to the premises; or 4) combinations of the above.

Administration of topical or systemic residual insecticides have become the preferred methods to eliminate flea infestations. Several of these insecticides have demonstrated their ability to very effectively control fleas on pets living in infested premises. Recent field studies have shown that the systemically active compounds afoxolaner, , fluralaner, lotilaner, , sarolaner, -selamectin, and spinosad can effectively control flea infestations, without the need for premise treatment. Flea infestations can be eliminated via regular monthly use of transdermal and oral-systemic approaches, because most fleas are killed before they can produce the next generation. However, even if the oral-systemic or transdermal insecticide used is 100% effective, control of an existing infestation will typically take 2–3 months because of the existing flea life stages in the environment.

Older, nonsystemic topical flea products, although frequently effective, can at times fail to eliminate an infestation. When this occurs, there can be several reasons for the failure:

  • Residual activity <100% within the labeled time frame

  • Rate of flea kill slows during the third or fourth week

  • Delayed or infrequent product reapplication

  • Simple underdosing

  • Mechanical removal of water-soluble insecticides during bathing or swimming

  • Development of insecticide resistance

Some of the currently available residual flea products may not be 100% effective against all cat flea strains between labeled reapplication periods because of genetic variability of different flea populations. Many of the factors that allow flea infestations to persist could possibly lead to genetic selection of resistant flea populations. Surviving fleas may be capable of producing viable eggs. Continued reproduction must be halted to prevent persistent flea infestations and selection for resistant fleas.

Reproduction can be prevented by administration of topical or systemic IGRs, which provide prolonged residual ovicidal activity, interrupting future flea development even after residual activity of an insecticide is diminished. Application of methoprene or pyriproxyfen to the hair coat of dogs and cats rapidly kills developing flea eggs in addition to residual ovicidal activity. The combination of fipronil/(S)-methoprene or other adulticidal/ovicidal products has demonstrated activity against adult fleas and provides prolonged residual ovicidal activity, thus reducing the potential for genetic selection of resistance. Not only have topically applied IGRs been shown to be ovicidal, but orally administered or injectable (cats only) lufenuron also provides ovicidal activity. Although not an IGR, selamectin also demonstrates ovicidal activity in cats.

Many pet owners mistakenly think that flea products either kill all newly acquired fleas within seconds to minutes or completely repel them. But repellency may sometimes be nonexistent, and residual products do not kill most fleas within minutes. Often, fleas may live for 6–24 hours and consume blood before being killed. Therefore, close scrutiny of treated pets in an infested environment occasionally results in a few flea sightings on pets for up to 8 weeks and occasionally longer until the infestation is eliminated. An additional complication for pet owners is infestation of the yard by wildlife, feral cats and dogs, or other infested pets. Often, owners will treat their pets but do not realize the environment their pet frequents may be constantly infested with fleas by wildlife or feral animals (especially cats). Even when pets go outdoors for only brief periods, they are susceptible to becoming infested. Additionally, people can act as carriers, bringing fleas into the household and infesting unprotected pets.

In cases of severe, massive flea infestations or severe pet or human flea allergy, treatment of the premises may be necessary. Pet owners should begin by conducting a mechanical control program. Helpful procedures include washing pet blankets, throw rugs, and pet carriers; in addition, pet sleeping and resting areas should be vacuumed thoroughly to help remove flea eggs and larvae. Seat cushions and pillows on sofas and chairs should be removed and vacuumed, and special attention given to crevices in sofas and chairs and to areas beneath sofas or beds where flea eggs and feces may drop from the pet and accumulate. Intermittent-light flea traps can also be beneficial. The overall goal of mechanical intervention is to reduce preexisting biomass of immature and adult life stages in the premises.

In addition, treatment of the premises with adulticides and IGRs may still be necessary in some infestations. Control may be achieved by using insecticides with residual activity (or by repeated application of short-acting insecticides) in combination with an IGR to prevent development of flea eggs and larvae. Methoprene and pyriproxyfen are the currently available IGRs for premise application. Insecticides and IGRs can be applied by broadcast treatment (hand pump sprayers or pressurized aerosols) or with total release aerosols or “foggers.” During application, the surface of all rugs and carpets must be treated adequately. Efforts should be directed to areas where flea eggs and larvae accumulate, such as carpets, cracks, grooves in hardwood floors, behind baseboards, under the edge of rugs, beneath furniture (beds, tables, and sofas), and within closets. In severe infestations, a second treatment may be necessary 7–10 days later because of continued emergence of adult fleas from cocoons hidden deep within carpets.

Elimination of fleas in the yard/garden can be an important aspect of flea control. Outdoor treatments (eg, imidacloprid, cyfluthrin, fenvalerate) should concentrate on primary areas of flea development, including protected microhabitats such as dog houses, within garages, under porches, and in animal lounging areas beneath shrubs or other shaded areas. Spraying flea control products over the large expanse of a shade-free lawn generally is not beneficial in control efforts and is poor environmental practice.

Despite the efforts of pet owners, the total elimination of fleas may not be feasible in some situations or may not occur rapidly enough to control clinical signs of FAD. Supportive medical treatment must be instituted to control pruritus and secondary skin disease in hypersensitive animals. Systemic glucocorticoids are often needed to control inflammation and associated pruritus. Short-acting prednisone or prednisolone can be administered initially at a dosage of 0.5–1 mg/kg per day, tapering the dosage and using alternate-day therapy until the lowest dose possible that still controls the pruritus is given. As soon as flea control is accomplished, the glucocorticoid can be discontinued. Anti-inflammatory therapy should never be used as a substitute for flea control.

Secondary bacterial skin infection can be associated with FAD. Systemic antimicrobials are commonly used to control the pyoderma and thus reduce the associated inflammation and pruritus. Selection of an appropriate antimicrobial should be based on bacterial cultures and results of antibiotic sensitivity tests. Hyposensitization consists of administering allergens to a hypersensitive animal on a regular basis in an attempt to obtain a state of clinical nonreactivity to flea bites. The effectiveness of currently available whole flea extracts is controversial.

Key Points

  • Many dog and cat owners expect that treatment will immediately eliminate a flea infestation. However, that is not possible due to the existing biomass residing within the premises, with complete elimination often taking 1–3 months.

  • Effective longterm flea control necessitates preventing fleas from reproducing.

  • Once the infestation is eradicated, most pet owners stop administering flea products. Lifelong flea control is highly recommended to prevent reinfestation.

For More Information

  • Companion Animal Parasite Council: Flea guidelines

  • Also see pet health content regarding flea allergy dermatitis in dogs and cats.


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